drm: make drm_legacy.h self-contained

Do not require users of drm_legacy.h to include other
files just to let it build
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20190526173535.32701-6-sam@ravnborg.org

drivers/gpu/drm/drm_legacy.h

@@ -29,11 +29,15 @@
  * drivers use them, and removing them are API breaks.
  */
 #include <linux/list.h>
+
+#include <drm/drm.h>
+#include <drm/drm_device.h>
 #include <drm/drm_legacy.h>
 
 struct agp_memory;
 struct drm_device;
 struct drm_file;
+struct drm_buf_desc;
 
 /*
  * Generic DRM Contexts